The invention concerns a method of and apparatus for decorating an uneven surface on an object which is stable in respect of shape, in particular for decorating a bottom of a beverage can.
In two-part aluminum or tin plate beverage cans available on the market nowadays, the actual Body portion of the can is produced from a single sheet metal portion in a stretching and drawing shaping process. Usually the bottom has a domed portion which goes into an annular ridge providing the surface on which the can body portion stands. The described three-dimensional profile of the can bottom makes it difficult to lacquer or print on that region of the can. In that respect there is also the need for the portion of the can which provides the surface on which it stands to be provided with a thicker coat of lacquer to take account of the higher mechanical loading in that region.
A bottom lacquering machine for such cans is known in the art, with which the entire can bottom is spray-lacquered with a solvent paint. Exact lacquer distribution is not possible with such a method. To provide the necessary abrasion resistance in the region of the bottom support surface additional lacquering of that surface is carried out after spray lacquering of the entire bottom. The described method of lacquering the bottom of a beverage can suffers however on the one hand from the disadvantage that the spray lacquering operation results in the formation of a lacquer mist which is also referred to as overspray and which causes an increased consumption of lacquer and requires suction removal measures to prevent that lacquer mist from being deposited at other locations on the can or to clean the air affected thereby. At the same time the described method also requires overlacquering of the body portion of the can as it is not possible to lacquer defined portions with a spray lacquering process, but only roughly differentiated portions. Spray lacquering the bottom of the can in addition also suffers from the disadvantage that solvent-bearing lacquers are generally used in that case, and they entail in particular pertinent environmentally related problems.
A further known method of lacquering the bottom of a beverage can includes three method steps, wherein the bottom support surface and the transitional region of the bottom to the peripheral surface of the can are lacquered by means of a roll application and the dome region of the bottom is lacquered by means of a spray application. Admittedly no overspray is produced by such a method, but the three process steps specified make the method comparatively complicated and expensive.
At any event with the known methods the bottom support surface of the can is passed through a roller lacquering means in a mass transport mode, a UV hardening lacquer generally being employed. Upon closer consideration of such a roller lacquering means however it is found that the contact pressure with which the application roller presses against the can and the bottom support surface of the can respectively must be very accurately controlled as otherwise the lacquer is forced laterally away from the center line of the bottom support surface. That causes a reduction in the thickness of lacquer on the center line of the bottom support surface where the aim is precisely to have a greater amount of lacquer. As the roller lacquering operation is carried out in mass transport of the cans, minor differences in the can height can occur and in addition the application roller cannot be held in every case parallel to the conveyor belt, the lacquer thicknesses which can be produced in the region of the bottom support surface can be kept within predetermined limits only with difficulty.